Atom-Probe Tomographic Investigation of Austenite Stability and Carbide Precipitation in a TRIP-Assisted 10 Wt Pct Ni St
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THE need to develop higher strength steels with superior blast and fragment protection for Naval applications than existing HSLA-100 steels[1,2] has attracted much attention in recent years.[3–9] Deformation due to the impact of high-velocity fragments from a blast can cause shear localization, which then leads to plastic shear instability and flow localization within thin adiabatic shear bands.[10–12] This results in shear plugging failure, in which material ahead of the projectile is
DIVYA JAIN is with the Department of Materials Science & Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208. Contact e-mail: [email protected] DAVID N. SEIDMAN is with the Department of Materials Science & Engineering, Northwestern University and also with the Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, IL 60208. ERIN J. BARRICK and JOHN N. DUPONT are with the Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015. Manuscript submitted July 13, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ejected as one solid-piece with little energy absorption.[11–13] To delay the onset of shear localization, the phenomenon of transformation-induced-plasticity (TRIP)[14–16] is utilized in newly developed low-carbon 10 wt pct Ni-Mo-Cr-V martensitic steels, referred to as 10 wt pct Ni steels herein.[6,17] Carbon concentration in these steels is held at a small concentration (~0.1 wt pct) to decrease the susceptibility of weld heat-affected zones (HAZs) to hydrogen cracking.[18,19] Nickel is used as a primary austenite stabilizer, as in the low-carbon 5.5 and 9 wt pct Ni steels, which have been in use for several decades for cryogenic applications.[20–31] Significant additions of Mo, Cr, and V in the 10 wt pct Ni steels results in the formation of alloy carbide precipitates,[17,32] which are not present in the 5.5 and 9 wt pct Ni cryogenic steels. Additions of these elements also result in very high hardenability of 10 wt pct Ni steels; Fonda et al.[33] studied the transformation behavior for a similar 9 wt pct Ni-Cr-Mo-V steel containing 0.11 wt pct C and reported a predominantly martensitic microstructure in the as-quenched condition for a wide range of cooling rates (130 C/s to 0.16 C/s).
10 wt pct Ni steels are optimally processed via a multistep Quench-Lamellarization-Tempering (QLT)treatment to form a fine dispersion of thermally stable Ni-enriched austenite (18 to 19 vol pct) in a tempered martensitic matrix.[6,17] In our companion article,[17] we elucidated the need to use a multistep QLT-treatment and described the factors controlling the thermal stability and kinetics of formation of Ni-stabilized austenite. It was established that the austenite formed after the T-step tempering remains thermally stable even at sub-zero temperatures. Zhang demonstrated that the austenite thus formed via QLTtreatment transforms to martensite during ballistic deformation.[6] Deformation-induced martensitic transformation of austenite leads to increased dynamic
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